High molecular weight fluorinated copolymers that are made up of copolymerized units of perfluoro(methyl vinyl ether) (PMVE) and either vinylidene fluoride (VF2) or tetrafluoroethylene (TFE) are known in the art and have been utilized to form vulcanized elastomer parts having relatively low glass transition temperatures (Tg), for example as disclosed in U.S. Patent Publication 2005/0215741 A1. The copolymers may be synthesized in the presence of chain transfer agents of the formula RfX, where Rf is a perfluorinated alkyl group and X is an iodine or bromine atom, thereby resulting in production of copolymers having iodine or bromine atom endgroups. It is also known to produce low molecular weight iodinated oligomers of PMVE and VF2 or TFE, for example as disclosed in co-pending U.S. Patent Application Publication 2009/0105435 A1.
Fluorosilicon polymers have also been prepared using similar low molecular weight oligomers. For example, U.S. Pat. No. 5,081,192 discloses the reaction of unsaturated silicon-containing compounds of Formula I, shown below, with iodinated reactants of Formula II to form fluorinated copolymers of Formula III. Such fluorinated iodinated copolymers may be converted into polymer networks by reaction with zinc.CH2═CY—(CH2)n—SiRxX3-x  (I)where X is a monovalent functional group, Y is a hydrogen atom or a lower alkyl group, R is a hydrogen atom or an inactive monovalent organic group, x is an integer of 0 to 3, and n is 0, 1, or 2PC—Im  (II)where PC is a polymer chain and m is a positive integer which is not larger than the number of ends of the polymer chain PC.PC—[CH2CYI—(CH2)nSiRxX3-x]m  (III)
Examples of suitable silicon-containing unsaturated compounds I disclosed in U.S. Pat. No. 5,081,192 include CH2═CHSiCl3, CH2═CHSiHCl2, CH2═CHSi(OC2H5)3, CH2═CHSi(OC2H5)2Cl, CH2═CHSi(OC2H5)Cl2 and CH2═C(CH3)SiCl3. Examples of the polymer chain (PC) of formula II that are disclosed include homopolymers or copolymers comprising at least one ethylenically unsaturated fluorinated compound such as TFE and compounds of the formulae C3F6 and CF═CFORf, where Rf is a perfluoroalkyl group having 1 to 16 carbon atoms. The polymer of formula III has a molecular weight of 5×102-5×106. When X in formula III is a halogenated atom, the iodo group(s) present can be removed by treating iodinated oligomer III with an alcohol and an element in the II or III group of the periodic table, preferably zinc. When exposed to air, the polymer forms siloxane bonds by dehydration condensation of silanols that are formed by hydrolysis of the SiRxX3-x groups, whereby the molecular weight increases and further three dimensional crosslinking occurs.
Crosslinked fluorosilicon polymer networks of the type described above are compositions having good sealing properties and low Tg. However, the processes disclosed in the prior art for producing such compositions are not optimal because crosslinking may occur prematurely before adequate network formation occurs. In addition, the presence of zinc in prior art polymer networks is problematic because of possible interference with some end use applications. For example, the presence of even low levels of zinc or other metals in fluoroelastomer seals used in semiconductor processing apparatus can cause contamination in parts produced using plasma etching techniques. It would be desirable to have available a process for production of two and three dimensional crosslinked networks of fluorinated polymers containing silicon wherein the polycondensation occurs by step-growth polycondensation. The resultant polymer network would then exhibit enhanced thermal and mechanical properties.